RT Journal Article SR Electronic T1 Pharmacokinetics and Metabolism of 5-125I-Iodo-4′-Thio-2′-Deoxyuridine in Rodents JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1671 OP 1676 VO 44 IS 10 A1 Toyohara, Jun A1 Gogami, Akie A1 Hayashi, Akio A1 Yonekura, Yoshiharu A1 Fujibayashi, Yasuhisa YR 2003 UL http://jnm.snmjournals.org/content/44/10/1671.abstract AB Pharmacokinetics and metabolite analysis of radioiodinated 5-iodo-4′-thio-2′-deoxyuridine (ITdU), a potential tumor cell proliferation-imaging agent, were investigated in rodents. Methods: Blood and urine metabolites of radiolabeled ITdU were analyzed by thin-layer chromatography in rats. Tissue fractionation studies in mice were also performed to investigate the incorporation of 125I-ITdU into DNA. Results: The blood distribution study suggested the influx of 125I-ITdU into rat peripheral blood cells. The blood metabolism of 125I-ITdU in vitro was negligible. Plasma analysis at 60 min after injection showed that 50% of the radioactivity was intact 125I-ITdU. The plasma clearance of 125I-ITdU showed a biexponential function with half-lives of 0.2 and 29.5 min, representing 94.2% and 5.8% of plasma 125I-ITdU concentration, respectively. 125I-ITdU was excreted in urine 86% intact at 3.5 h after injection. Extraction of the tissues demonstrated that >70% of the activity in the duodenum, spleen, and thymus was incorporated into DNA at 8 h after injection. On the other hand, <8% of the activity in the liver was in the DNA fraction at that time. Conclusion: These data demonstrate that 125I-ITdU is less susceptible against catabolism and readily incorporated into DNA in proliferating tissues. Therefore, 125I-ITdU may be useful in measuring DNA synthesis with SPECT.